The most recent statistic I was able to find for Americans living below the poverty line in the 1950s places the rate at 22.4%, more than one in five. Keep in mind this was before there were anti-poverty programs such as food stamps, welfare and education programs. These came into being largely during Lyndon Johnson's administration in the 1960s and his War on Poverty programs.
These programs, while expensive and not addressing the root causes of poverty very directly, cut the overall poverty rate in half, to just over 11%. Since that time it has climbed again to nearly one in six Americans today.